A. Technical Field
The present invention relates to AC-DC adapters, and more particularly, to systems, devices and methods of employing a load detector to detect a load condition of the AC-DC adapters based on transient variation of a DC output voltage, and therefore, enable the AC-DC adapters to switch from a low power mode to a normal power mode.
B. Background of the Invention
AC-DC adapters are power supplies commonly employed to convert an alternating current (AC) to a direct current (DC) for powering various electronic devices. A typical AC-DC adapter is based on a linear power supply or a switched-mode power supply. Originally most AC-DC adapters were linear power supplies comprising a transformer, a rectifier and a filter. The transformer directly down-converts the incoming AC signal to a lower voltage AC signal at the same frequency prior to rectification and filtering for generating a satisfactory DC voltage level which may have a negligible ripple variation. Due to the relatively low frequency of 50 Hz or 60 Hz, the transformer was relatively large. The bulky transformer not only introduces losses but also fundamentally limits the adapter from being integrated into wall outlets. Moreover, a resistive divider configuration is adopted to maintain a stable DC output voltage from the linear power supplies. Significant power loss as heat causes low energy efficiency even when the linear power supply does not drive a load.
To address the issues of physical size and power efficiency, switched-mode power supplies have recently been employed. In a typical switched-mode power supply, the frequency of the incoming AC signal is first up-converted to a higher frequency, and as a result, both the size and the power loss of the transformer are reduced. Moreover, pulse width modulation rather than a resistive divider configuration is employed to regulate the output voltage in a switched-mode power supply. The overall energy efficiency of a switched-mode power supply doubles that of a linear power supply.
The upcoming Energy Star 2013 will impose more stringent challenges on the energy efficiency of power supplies. Energy efficiency has already been enhanced from 30-40% to the range of 65% to 80% by using switched-mode power supplies rather than linear power supplies. However, to further enhance energy efficiency, a sleep mode or a low power mode is mandated when any power supply is first connected to wall sockets or when no DC load is applied.
When the switched-mode power supply is applied to drive a DC load such as a notebook computer, a data pin is employed to identify the presence of load so that the AC-DC adapter may switch between power modes based on the load condition. FIG. 1A illustrates an exemplary block diagram 100 for an AC-DC adapter 110 coupled to a DC-powered device (a DC load) 160. The AC-DC adapter 110 consists of a switched-mode power supply (SMPS) 102 and a power supply identification circuit 104. The cable 112 connecting the AC-DC adapter 110 to a DC load 160 (e.g., the notebook computer) is a co-axial, plug-in-type 3-pin connector including a positive terminal 118 (i.e., VOUT wire), a negative terminal 120 (i.e., ground wire), and a center data pin 122 (i.e., Load Presence/ID). The center data pin 122 is used to communicate power supply identification (PSID) information to the DC load 160. An auto-sensing circuitry in the PSID circuit 104 is configured to detect non-existence of the DC load 160 by monitoring the absence of PSID signals communicated from the DC load 160 to the PSID circuit 104. The Load Presence/ID signal is generated to indicate load condition and control a power mode control circuit 106 that further enables a proper power mode in the switched-mode power supply 102.
The aforementioned AC-DC adapter starts up at the low power mode every time it is plugged into a wall socket. The PSID circuit is used to detect the DC load and control the SMPS to switch from the low power mode to a normal power mode. This approach requires a data cable to connect the AC-DC adapter 110 and the DC load 160 in addition to two DC power terminals. When the data pin 122 is inadvertently damaged, the AC-DC adapter may never initiate the normal power mode to drive the DC load properly. A serial resistor may be placed in the VOUT path between the AC-DC adapter 110 and the DC load 160 to monitor the load condition. A low impedance resistor has to be used to minimize its power consumption, and as a result, a highly sensitive readout circuit has to be applied to detect any variation of the voltage drop on the serial resistor. An alternative solution is needed to effectively detect the load condition and start the normal power mode properly.